This invention relates to techniques for converting an oxygenated aliphatic compound, such as methanol, to lower olefins. In particular, it provides a continuous process for producing an olefinic product rich in C.sub.2 -C.sub.5 alkenes, especially C.sub.4.sup.+ alkenes. In view of the availability and low cost of synthetic methanol (MeOH), primary emphasis is placed on this feedstock material in following description of the methanol-to-olefin (MTO) process.
Processes for converting lower oxygenates such as methanol to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroleum origin. In particular, they provide a way by which methanol can be converted to a major amount of C.sub.2 -C.sub.5 olefins and a minor amount of gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes, such as steam reforming or partial oxidation to make the intermediate syngas. Crude methanol from such processes usually contain a significant amount of water, usually in the range of 4 to 20 wt. %; however, the present invention is useful for removing water in lesser amounts or greater.
Various zeolitic catalysts are useful for converting methanol and other lower aliphatic alcohols or corresponding ethers to olefins. Recent interest has been directed to a catalytic process for converting methanol over ZSM-5 and related catalysts to valuable hydrocarbons rich in ethene and C.sub.3.sup.+ alkenes. Various processes are described in U.S. Pat. Nos. 3,894,107 (Butter et al); 3,928,483; 4,025,575; 4,252,479 (Chang et al); 4,025,572 (Lago); 4,328,384 (Daviduk et al); and 4,547,616 (Avidan et al); incorporated herein by reference. It is generally known that MTO processes can be optimized to produce a major fraction of C.sub.2 -C.sub.4 olefins; however, a significant C.sub.5.sup.+ byproduct may be coproduced. Prior process techniques for increasing lower olefin selectivity have provided for controlled deposition of coke byproduct on the catalyst surface .
Methanol may be first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed at elevated temperature and pressure over a catalyst such as ZSM-5 zeolite for conversion to the hydrocarbon products . Water may be removed from the methanol dehydration products prior to further conversion to hydrocarbons and the methanol can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of excess water vapor at the reaction temperatures employed; but this step is not essential.
Typically the crude methanol feestock employed in MTO processes contains about 4 to 20 wt. % water as the principal impurity. Excessive water not only contributes to catalyst deactivation, but also requires larger volume equipment to handle the increased throughput. Various proposals have been put forth for reducing the water content of crude methanol, for instance the distillation system described by Mao et al in copending U.S. patent application Ser. No. 823,153, filed Jan. 27, 1986, incorporated by reference.
It is main object of the present invention to provide a novel and economic technique for removing excess water from crude MTO feedstocks, including novel operating methods and equipment for treating these oxygenate feedstocks.